have developed a sensor-packed dental retainer that people with high-level spinal cord injuries can manipulate with their tongue to manoeuvre an electric wheelchair.

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The latest prototype of the Tongue Drive System lets users wear a dental retainer embedded with sensors that track the location of a tiny magnet attached to the tongues of users. This is achieved by giving participants a "clinical" tongue piercing and tongue stud containing a tiny magnet embedded in the upper ball.

In previous iterations of the system, the sensors that tracked the movement of the magnet on the tongue were mounted on a clunky headset worn by the user. However, the headset would shift on the user's head and the system would need to be re-calibrated. "By moving the sensors inside the mouth, we have created a Tongue Drive System with increased mechanical stability and comfort that is nearly unnoticeable," said Maysam Ghovanloo, an associate professor in the School of Electrical and Computer Engineering at the Georgia Institute of Technology.

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The mouth device contains magnetic field sensors mounted on its four corners to detect the moment of the tiny magnet attached to the tongue. It is powered by a lithium-ion battery and has an induction coil to recharge it. The retainer sits against the roof of the mouth and is protected with a water-resistant material.

Ghovanloo explained: "Because the dental appliance is worn inside the mouth and moulded from dental impressions to fit tightly around an individual's teeth with clasps, it is protected from these types of disturbances."

The output signals from the sensors are wirelessly transmitted to an iPod Touch or iPhone. Software installed on the Apple device works out the relative position of the magnet with respect to the array of sensors in real time and interprets the user's commands. This information is then used to control the movements of a cursor on the computer screen or to substitute for the joystick function in a powered wheelchair.

The team has also created a universal interface for the intraoral Tongue Drive System that attaches to a standard electric wheelchair. It holds the iPod, wirelessly receives the sensor data and delivers it to the iPod touch, charges it and features a container for leaving the retainer overnight for charging.

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The system can be trained with multiple commands -- unlike the common sip-n-puff device that acts as a switch controlled by sucking or blowing through a straw.

The team are going to test the usability of the Tongue Drive System using able-bodied individuals before moving onto clinical trials with people with high-level spinal cord injuries.